Computer Network

TCP/IP

  1. 为什么 ack=seq+1 而不是 ack=seq? 通过查询资料和所学内容，我们知道： 1. 三次握手的前两次握手，client和server会根据一定的算法(连接四元组和时间戳还有Security Number)生成ISS。 2. 三次握手的后两次握手，client和server分别会对对方发过来的SYN进行应答，在ack序列上等于seq+1。 3. 后面传输数据的时候，sender发送len长度的data，receiver就会将ack(receiver)=seq(sender)+len(sender)。

    所以说，3中接收到多少数据就讲多少数据长度的len加在seq上是一个正常的行为，问题出在2的确认SYN中为什么将ack=seq+1，搜索到结果有如下几个： 1. SYN和FIN也是需要确认的信息，所以需要占据一个seq，表示该信息已经被确认，不需要重传。 2. 如果SYN和FIN不占据一个seq，对于FIN而言，四次挥手的第二三次server的挥手除了FLAG外其他其实是一样的，即一样的ack，可能也有一样的seq(没有数据需要传输的情况下)，可能导致client段无法分辨第二次挥手的ACK和第三次挥手的FIN。

    Notice that the acknowledgement number has been increased by 1 although no payload data has yet been sent by the client. This is because the presence of the SYN or FIN flag in a received packet triggers an increase of 1 in the sequence. (This does not interfere with the accounting of payload data, because packets with the SYN or FIN flag set do not carry a payload.)

    注：收到的报文中只有ACK时不需要将seq+1。

  2. TCP传输中ACK的优化 已经知道的有延迟ACK，还有就是ACK和需要传输的数据一起过去，SO上面已经有大佬画了图：

    Before ACK Optimization:   
 +-------------------------------------------------------+
 |     client           network            server        |
 +-----------------+                +--------------------|
 |    (connect)    | ---- SYN ----> |                    |
 |                 | <-- SYN,ACK -- |     (accepted)     |
 |   (connected)   | ---- ACK ----> |                    |
 \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/

 when client sends...
 \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/
 |                 |                |                    |
 |     (send)      | ---- data ---> |                    |
 |                 | <---- ACK ---- |  (data received)   |
 \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/

 when server sends...
 \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/
 |                 |                |                    |
 |                 | <--- data ---- |       (send)       |
 | (data received) | ---- ACK ----> |                    |
 \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/

 ...and so on, til the connection is shut down or reset

 =========================================================
 =========================================================
 After ACK Optimization:
 \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/
 |                 |                |                    |
 |                 | <--- data ---- |       (send)       |
 | (data received) |                |                    |
 |     (send)      | -- data,ACK -> |                    |
 |                 |                |  (data received)   |
 |                 | <- data,ACK -- |       (send)       |
 | (data received) |                |                    |
 |  (wait a bit)   | <--- data ---- |       (send)       |
 | (data received) |                |                    |
 |     (send)      | -- data,ACK -> |                    |
 |                 |                |  (data received)   |
 |     (send)      | ---- data ---> |   (wait a bit)     |
 |                 |                |  (data received)   |
 |                 | <- data,ACK -- |       (send)       |
 | (data received) |                |                    |
 |  (wait a bit)   |   (dead air)   |                    |
 |                 | ---- ACK ----> |                    |
 \_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/\_/

  3. TCP Connection State Diagram in rfc793 

                               +---------+ ---------\      active OPEN
                           |  CLOSED |            \    -----------
                           +---------+<---------\   \   create TCB
                             |     ^              \   \  snd SYN
                passive OPEN |     |   CLOSE        \   \
                ------------ |     | ----------       \   \
                 create TCB  |     | delete TCB         \   \
                             V     |                      \   \
                           +---------+            CLOSE    |    \
                           |  LISTEN |          ---------- |     |
                           +---------+          delete TCB |     |
                rcv SYN      |     |     SEND              |     |
               -----------   |     |    -------            |     V
+---------+      snd SYN,ACK  /       \   snd SYN          +---------+
|         |<-----------------           ------------------>|         |
|   SYN   |                    rcv SYN                     |   SYN   |
|   RCVD  |<-----------------------------------------------|   SENT  |
|         |                    snd ACK                     |         |
|         |------------------           -------------------|         |
+---------+   rcv ACK of SYN  \       /  rcv SYN,ACK       +---------+
|           --------------   |     |   -----------
|                  x         |     |     snd ACK
|                            V     V
|  CLOSE                   +---------+
| -------                  |  ESTAB  |
| snd FIN                  +---------+
|                   CLOSE    |     |    rcv FIN
V                  -------   |     |    -------
+---------+          snd FIN  /       \   snd ACK          +---------+
|  FIN    |<-----------------           ------------------>|  CLOSE  |
| WAIT-1  |------------------                              |   WAIT  |
+---------+          rcv FIN  \                            +---------+
| rcv ACK of FIN   -------   |                            CLOSE  |
| --------------   snd ACK   |                           ------- |
V        x                   V                           snd FIN V
+---------+                  +---------+                   +---------+
|FINWAIT-2|                  | CLOSING |                   | LAST-ACK|
+---------+                  +---------+                   +---------+
|                rcv ACK of FIN |                 rcv ACK of FIN |
|  rcv FIN       -------------- |    Timeout=2MSL -------------- |
|  -------              x       V    ------------        x       V
 \ snd ACK                 +---------+delete TCB         +---------+
  ------------------------>|TIME WAIT|------------------>| CLOSED  |
                           +---------+                   +---------+

Reference

  1. StackOverflow | Does TCP send a SYN/ACK on every packet or only on the first connection?

  2. Understanding TCP Sequence and Acknowledgment Numbers

  3. Why does an pure ACK increment the sequence number?

  4. RFC 793

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